The goal of this project is to characterize the enzyme in human plasma that catalyzes the hydrolysis of acetate from 1-0-alkyl-sn-glycero-3-phosphocholine ("platelet-activating factor";PAF). This phospholipid is a potent activator of platelets and neutrophils, and has marked hemodynamic and hepatic effects. These various effects suggest that the concentration of PAF in blood must be precisely regulated to prevent dramatic consequences. This conclusion is supported by animal studies in which an infusion of PAF caused anaphylaxis, shock, and death, and in models of anaphylaxis where PAF was shown to appear in the blood. Conversely, others have proposed that PAF is a normal, endogenous antihypertensive compound. These possibilities are not mutually exclusive, but if both are operative would suggest a need for precise control. The products of the acetylhydrolase reaction are without biological activity, which suggests that this enzyme can play a key role in such regulation. The initial aim of this project is to complete the purification of the PAF acetylhydrolase from human plasma. This will allow us to characterize its physical properties, including the mechanism of its interesting association with low density and high density lipoproteins. The catalytic properties, including the preferred physical state of the hydrophobic substrate, will be defined. Another major aim is the preparation of antibodies to the acetylhydrolase for use in studies of its cellular source, to determine whether post-translational processing occurs, to examine the regulation of secretion, and the mechanism of association with lipoproteins. In preliminary experiments it has been found that plasma accelerates the degradation of cell-associated PAF. A set of proposed experiments will determine whether this phenomenon is due to the activity of plasma PAF acetylhydrolase and whether it requires interaction with a cell surface lipoprotein receptor. Finally, the activity of acetylhydrolase will be determined in the plasma of patients with diseases that are plausibly related to PAF. These studies should thoroughly characterize this enzyme, provide insight into its physiological (and perhaps pathological) role, and suggest ways for altering activity, if appropriate.